1. Field of the Invention
This invention relates generally to systems and methods for evaluating thermal bonds.
2. Description of the Related Art
Many mechanical and electrical devices generate heat internally which must be dissipated to the environment to keep the devices within a range of desired operating temperatures. Such heat-producing devices may include engines, bearings, motors, power supplies, transformers, amplifiers, control modules, and computer chips and components including graphics controllers, network interfaces, and central processing units (CPUs). In some applications it is desirable to transfer heat from a heat-producing device to a heat-absorbing apparatus so that the heat may be used for useful purposes. These useful purposes may include generating electricity, providing heat for industrial processes, heating water, heating air (e.g. to heat the occupied space in buildings), or preventing freezing. Heat-absorbing apparatuses may comprise fluids which move, change phase, or both, to transfer heat, and may involve or approximate thermodynamic cycles such as a Rankine, Carnot, or Brayton cycle. On the other hand, heat-absorbing apparatuses may be simple solid devices such as heat sinks (heatsinks), for instance, typically without macroscopic moving parts.
A heat sink is one type of heat-absorbing apparatus. As an example, heat sinks have been used to cool CPUs in computers including general-purpose desk-top PCs. A heat sink is a device, typically monolithic, that conducts and usually dissipates heat. Heat sinks may be made of a metal such as aluminum or copper, or in some applications may be made of other materials such as ceramic or plastic. The heat typically travels from the heat-generating device to the heat sink primarily through conduction, and then travels through the heat sink, typically also via conduction. Heat sinks may have a large surface area to dissipate heat (e.g. large relative to the surface area of the heat-producing device), generally through convection, e.g. to surrounding air. The high surface area may be accomplished with fins, holes, hills and valleys, or other geometric features. The air may be blown with a fan to increase the Nusselt number and improve cooling, or the system may rely on natural convection. Some heat may also be transferred through radiation, particularly in high-temperature applications, and heat sinks may be configured with a surface having a high emissivity to facilitate radiant heat transfer. For instance, heat sinks may have black coatings.
Heat sinks may comprise multiple parts, e.g. multiple fins attached to the heat-producing device. In addition, e.g. in applications where heat is produced transiently, heat sinks may not have fins or other features to dissipate heat, but may rely on their bulk to absorb and store the heat produced, which may then be dissipated slowly over time. Heat sinks may also perform other functions, including acting as a structure or enclosure, or may be formed from components also used for other purposes. For instance, the housing of a distributor in an automobile may serve as a heat sink for an ignition control module (ICM) housed within the distributor. In such an application, the heat produced by the ICM may typically transfer by conduction into the distributor housing, and then by convection to the air traveling through the engine compartment of the automobile.
In order to conduct heat effectively, in many applications it is desirable to have a good thermal bond (i.e. a low thermal resistance) between the heat-producing device and the heat-absorbing apparatus so that heat will transfer relatively freely from the heat-producing device to the heat-absorbing apparatus. The quality of the thermal bond may be more important where the heat-producing device is small relative to the amount of heat that is generated within, or where the heat-producing device must be maintained at a temperature close to ambient. In typical applications where the heat-producing device and heat-absorbing apparatus are separate components and heat is transferred between them by conduction, a heat-conducting substance such as a thermal grease or thermal paste may be used between the heat-producing device and the heat-absorbing apparatus. A heat-conducting substance is typically a non-Newtonian fluid that may be tacky and flexible, at least when installed, so that it fills most of the microscopic gaps between the surfaces of the heat-producing device and heat-absorbing apparatus. The heat-conducting substance may be an adhesive or glue that holds the heat-producing device and heat-absorbing apparatus together once assembled. The heat-conducting substance may be a metal such as solder, or a thermal wax, and may be melted during the joining of the heat-producing device and heat-absorbing apparatus, but may remain solid at the normal operating temperature of the heat-producing device. A heat-conducting substance is preferably a good conductor of heat, and may be an electrical conductor so that heat conduction may occur via the movement of electrons. The surfaces of the heat-producing device and heat-absorbing apparatus may be cleaned prior to applying the heat-conducting substance in order to avoid thermal resistance from foreign materials on the surfaces, such as oxidation.
In one specific application, INTEL PENTIUM-based computers may have a heat sink attached to the top of the CPU chip which are designed to dissipate heat produced by the chip. The heat is typically dissipated to air that may be moved by fans and may ultimately be vented to the outside of the equipment case. It is typically important that the CPU be kept below a temperature that would shorten its life, cause mechanical damage, cause software to malfunction, or destroy the device completely. Systems and methods have been developed to test and evaluate the effectiveness of heat sinks, including systems and methods that use thermal sensors such as thermal diodes, which may be on-board components of the CPU chips. A properly rated heat sink with sufficient airflow can perform adequately if it is properly affixed to the CPU chip.
In many applications, it is desirable that a good thermal bond be produced between a heat-producing device and a heat-absorbing apparatus. However, as with any manufacturing process, it is difficult or expensive to verify that all items manufactured have a good thermal bond. This is particularly true where the heat-producing device has a small surface area for heat conduction, such as a flip-chip (also written flip chip or flipchip) CPU. Therefore, it would be desirable to have a convenient system and process or method to test or evaluate the thermal bond between a heat-producing device and a heat-absorbing apparatus.
For instance, it is desirable to have a convenient system and method to test the thermal bond between a computer CPU and its heat sink. During the manufacture of a computer, a heat sink may be affixed to the CPU and a path of low thermal conductivity established with a combination of thermal paste or adhesive and pressure. There is typically a relatively small area on the top of the chip that may need to have a low resistance path to the heat sink and modifications to this area (e.g. drilling to insert a temperature probe) may dramatically affect its operation. Due to the mechanical arrangement of the two parts (CPU and heat sink) as well as the complexities encountered when trying to remove heat from CPUs in confined spaces, in the past it has been difficult to verify during manufacture that the thermal bond between the CPU and the heat sink has been correctly made. Therefore, as a specific example, it would be desirable to have a convenient system and process or method to test or evaluate the thermal bond between a CPU and a heat sink.
This invention provides a system and method for evaluating the thermal bond between a heat-producing device and a heat-absorbing apparatus. In an exemplary embodiment, it provides a convenient system and process or method to test or evaluate the thermal bond between a CPU and a heat sink. Features of this invention include that it is easy and convenient to use, that in some applications it requires few or no additional parts, and that costs are therefore minimal.
In furtherance of these features, this invention provides a system for evaluating the thermal bond between a heat-producing device and a heat-absorbing apparatus which may be a heat sink. A thermal bond with a thermal resistance generally exists between the heat-producing device and the heat-absorbing apparatus. In some embodiments, a thermal sensor measures the temperature at or near the device, and a circuit or CPU is configured to perform the functions of operating the device substantially at a first power level, measuring a first temperature with the thermal sensor, operating the device substantially at a second power level, measuring a second temperature with the thermal sensor, and calculating the thermal resistance using the first temperature and the second temperature. The heat-producing device may be the circuit, and the thermal sensor may be located within the circuit. The circuit may be configured to calculate the thermal resistance by subtracting the second temperature from the first temperature, and in some embodiments, by dividing the difference by the first power level. The thermal bond may utilize a heat-conducting substance which may be thermal grease, thermal paste, thermal wax, glue, adhesive, or solder. Furthermore, the second temperature may be measured a predetermined time after the second power level is initiated.
The present invention also provides a method of evaluating the thermal bond between a heat-producing device and a heat-absorbing apparatus. In one embodiment the method includes, initiated in the following order, the steps of operating the device substantially at a first power level, measuring a first temperature of or near the device, operating the device substantially at a second power level, measuring a second temperature of or near the device, and calculating the thermal resistance of the thermal bond between the device and the apparatus. The calculating procedure may include use of at least the first temperature and the second temperature. In fact, the second power level may be less than the first power level, and the calculating procedure may involve subtracting the second temperature from the first temperature. The step of operating the device substantially at a first power level may involve operating the device until the heat-producing device and the heat-absorbing apparatus substantially reach equilibrium temperature. In addition, the step of measuring the second temperature may occur at least 10 seconds after the initialization of the step of operating the device at the second power level. In addition, the method according to the present invention may also include the step of accepting the thermal bond if the thermal resistance is below a threshold.
The present invention further provides a method of evaluating the thermal bond between a heat-producing device and a heat sink. In one embodiment, the method includes the steps of operating the device substantially at a first power level, at least until the device and the heat sink substantially reach equilibrium temperature, measuring a first temperature substantially of the device, operating the device substantially at a second power level, typically less than the first power level, and after a period of time at the second power level, measuring a second temperature, also substantially of the device. The method also usually includes calculating the thermal resistance of the thermal bond between the device and the heat sink, which may involve subtracting the second temperature from the first temperature. The method may also include the step of accepting the thermal bond if the thermal resistance is below a threshold. The device may be an integrated circuit chip, such as, for example, a CPU, and the measuring may involve using a thermal diode, which may be integral with the device.
In addition, the period of time before the second measurement is made may be a predetermined amount of time between the beginning of the operating the device at the second power level and the measuring of the second temperature. The method may further include the step of remounting the heat sink on the device if the thermal resistance is above the threshold. Remounting may involve the steps of separating the heat sink and the device, cleaning the heat sink and the device, applying or reapplying a heat-conducting substance, and reattaching the heat sink and the device. The calculating procedure may be performed by the device, and may involve dividing by the power consumption of the device. The thermal bond may utilize thermal grease, thermal paste, thermal wax, or adhesive. The second power level may be less than 10% of the first power level, and the CPU may be a flip-chip CPU. Furthermore, some embodiments include a step of measuring a third temperature at or near the device, typically measured after the second temperature, but while the device is still being operated at the second power level.
The present invention even further provides products made according to the above methods.